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解題思路是將b+c看做整體,化簡,運用余弦定理. (a+b+c)(b+c-a) =(b+c)^2-a^2 =b^2+c^+2bc-a^2=3bc ∴a^2=b^2+c^2-bc 由任意三角形余弦定理可知:a^2 = b^2 + c^2 - 2·b·c·cosA ∴cosA=1/2 ∵A為三角形內角 ∴A=60°
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